Batteries, which generate electrical energy by way of electrochemical oxidation and reduction, are used over a wide range of applications. For example, the batteries are used in gradually expanding fields, including: devices carried around in user's hands, such as portable phones, laptop computers, digital cameras, video cameras, tablet computers, electrically-driven tools, and so on; electrically-driven power apparatuses such as electric bicycles, electric auto-bicycles, electric vehicles, hybrid vehicles, electric boats, electric airplanes, and so on; power storage apparatuses for use in storing electricity generated from renewable energy or surplus generated electricity; or uninterruptable power supply to stably supply electricity to a variety of information communication apparatuses including server computers, base stations for communication, and so on.
The battery includes three basic elements which are: a negative electrode containing a material that undergoes oxidation and releases electrons during discharging; a positive electrode containing a material that undergoes reduction and accepts electrons during discharging; and an electrolyte that allows migration of operating ions between the negative electrode and the positive electrode.
The battery can be categorized into a primary battery which is not reusable once it is discharged, and a secondary battery which has at least partially reversible electrochemical reaction, thus are repetitively chargeable and dischargeable.
For the secondary battery, lead-acid battery, nickel-cadmium battery, nickel-zinc battery, nickel-iron battery, silver oxide battery, nickel metal hydride battery, zinc-manganese oxide battery, zinc-bromide battery, metal-air battery, lithium secondary battery, and so on, are known.
Among these, the lithium secondary battery is attracting greatest commercial attentions, in view of its relatively higher energy density, higher battery voltage and longer storage life than the other secondary batteries.
Concerning the secondary battery, materials used for positive electrode material and negative electrode material have critical influence on the performance of the secondary battery. Accordingly, a variety of efforts are being made to provide the positive electrode material and the negative electrode material which have stability at high temperature and can provide high energy capacity and low manufacture cost.
However, it will not be easy to develop a positive electrode material and a negative electrode material that have superior performances in all aspects. Therefore, attempts are recently made to make up for the shortcomings of respective secondary batteries by way of connecting in parallel the secondary batteries that include different types of positive electrode materials and negative electrode materials from each other.
Hereinafter, the secondary battery formed by connecting different types of secondary batteries in parallel will be referred to as a ‘hybrid secondary battery’.
Meanwhile, the hybrid secondary battery often has a voltage profile that includes a point of inflection, when the constituent batteries have different operating voltage ranges from one another. This is because, when the constituent batteries have different operating voltage ranges, dominant reaction kinetics will change during charging or discharging of the hybrid secondary battery.
When the voltage profile of the hybrid secondary battery has a point of inflection, the voltage variation near the point of inflection is relatively smaller compared to state of charge (SOC) variation. That is, voltage seldom changes even when the state of charge considerably changes. Meanwhile, voltage measured by a sensor has an error due to sensor error, dynamically-changing polarization voltage, and so on. Accordingly, it is hard to ensure accuracy when a hybrid secondary battery is controlled by directly using the measured voltage near the point of inflection. This is because, near the point of inflection, a general battery control system estimates a considerable change in the electrochemical state of a battery, even with a slight variation in the measured voltage.
Accordingly, a method is necessary for a hybrid secondary battery, which can indirectly estimate the voltage using other measurable parameters, instead of directly measuring the voltage.